Here are my top 40 Drupal modules that seem to happen more often I am using them in my Drupal projects.

 1. addtoany
 2. blocks404
 3. boost
 4. cacherouter
 5. canonical_url
 6. captcha
 7. cck
 8. custom_breadcrumbs
 9. date
10. diggthis
11. filefield
12. gravatar
13. htmlpurifier
14. image_caption
15. image_fupload
16. imageapi
17. imagecache
18. imagecache_profiles
19. imagefield
20. imagefield_crop
21. imagefield_tokens
22. imce
23. imce_wysiwyg
24. menu_breadcrumb
25. montharchive
26. nodewords
27. page_title
28. pathauto
29. seo_checklist
30. tagadelic
31. taxonomy_breadcrumb
32. taxonomy_manager
33. taxonomy_menu
34. taxonomy_vtn
35. token
36. twitter
37. views
38. workflow
39. wysiwyg
40. xmlsitemap